1. Field of the Invention
The present invention relates to a semiconductor photodetector of the type that receives incident light on its top surface, and more particularly to a semiconductor photodetector adapted to have an increased incident light absorption efficiency and increased response speed.
2. Background Art
When a general semiconductor photodetector is operated to receive light, a reverse bias is applied to its PN junction to form a depletion layer. The light incident through the light receiving surface of the semiconductor photodetector is absorbed by its light absorption layer, which is a region with a narrow energy band, resulting in the generation of carriers within the absorption layer. Since the depletion layer is formed in the absorption layer, the carriers generated by the light absorption are accelerated by the depletion layer. It should be noted that the flow of these carriers results in an electric current component.
There is a need to improve the light absorption efficiency of such semiconductor photodetectors, since this efficiency primarily determines the efficiency of converting the incident light into carries. That is, the light absorption efficiency of a semiconductor photodetector is a measure for indicating the quantity of carriers generated within its depletion layer when the photodetector receives a given amount of incident light. The higher the absorption efficiency, the better, since the incident light-to-current conversion efficiency increases with increasing absorption efficiency. [Techniques for improving the light absorption efficiency are disclosed in Japanese Laid-Open Patent Publication Nos. 1-205573 (1989), 2005-159002, 2003-179249, and 2004-241533.]
Further, there is also a need to increase the response speed of semiconductor photodetectors in order to accommodate high speed modulation. The response speed of a semiconductor photodetector is the time required for the photodetector to generate an electric current after receiving incident light on its light receiving surface. It should be noted that if the current generated in the semiconductor photodetector includes a delayed component, it may prevent the photodetector from accommodating high speed modulation. [A technique for improving the response speed is disclosed in Japanese Laid-Open Patent Publication No. 4-116977 (1992).]
It is common for semiconductor photodetectors to have top and bottom electrodes formed on their top and bottom surfaces, respectively. It should be noted that the top surface is designed to be a light receiving surface. FIGS. 14 and 15 show an exemplary semiconductor photodetector having such top and bottom electrodes. Specifically, FIG. 14 is a plan view as viewed from the top surface of the semiconductor photodetector. In FIG. 14 a top electrode 100 is shown which is formed on an insulating film 102. FIG. 15 is a cross-sectional view taken along line 15-15 of FIG. 14. Referring to FIG. 15, a first light transmissive layer 104 of a second conductivity type and a second light transmissive layer 106 of a first conductivity type are formed under the insulating film 102. Both the first and second light transmissive layers 104 and 106 allow light to pass through. A light absorption layer 108 of the first conductivity type is formed under the first and second light transmissive layers 104 and 106. A semiconductor substrate 110 of the first conductivity type underlies the light absorption layer 108. Further, a bottom electrode 112 is formed on the bottom surface of the semiconductor substrate 110.
In this semiconductor photodetector, it is necessary to decrease the distance between the top electrode 100 and the bottom electrode 112 in order to reduce the driving bias voltage and thereby reduce the power consumption. FIG. 16 shows a semiconductor photodetector with a thinned light absorption layer when a driving bias voltage is applied between the top electrode 100 and the bottom electrode 112 of the photodetector. Referring to FIG. 16, the applied driving bias causes a depletion layer 120 to be formed between the first light transmissive layer 104 and the second light transmissive layer 106 and between the first light transmissive layer 104 and the light absorption layer 108. When the semiconductor photodetector with the depletion layer 120 formed therein receives incident light, 116, through its light receiving surface, the light absorption layer 108 absorbs it, thereby generating carriers therein. The depletion layer 120 then accelerates these carriers, thereby generating an electric current component.
As can be seen from the foregoing description, increasing the number of carriers generated in the semiconductor photodetector requires an increase in the thickness of the light absorption layer 108 to increase the percentage of the light absorbed in the layer. However, an increase in the thickness of the light absorption layer 108 means an increase in the travel distance of the generated carriers, resulting in reduced response speed of the semiconductor photodetector.
Referring now to FIG. 17, it happens that some of the light (122) incident on the light receiving surface passes through the light absorption layer 108 without being absorbed. The light, 122, that has transmitted through the light absorption layer 108 is reflected by the bottom electrode 112, etc. The reflected light 122 may be absorbed by the light absorption layer 108 at a location away from the depletion layer 120. In such a case, the carriers generated by this absorption take a certain time to reach the depletion layer 120 and result in an electric current component. That is, this current component is delayed from those resulting from the carriers generated when light is absorbed by the light absorption layer 108 at locations in or near the depletion layer 120. It should be noted that a semiconductor photodetector cannot achieve high response speed if its photoelectric current includes such a delayed component.